Black Spruce Wind Throws

published by Bruce Hayden on Tue, 2013-09-03 16:10

Pressure Gradient Winds

Coldfoot Black Dateline: Coldfoot, Alaska from the haul-road bus. Boy, the black spruce here on the southern slopes of the Brooks Range has a wimpy root base! Upturned tress, and there weren't all that many, weigh for a lack of windstorms. Could this be?

Atlas check. Fairbanks: mean annual wind speed 5 mph with an average monthly range from 3 to 7 mph. That is less than half of the average in the lower 48. Along the arctic coast of Alaska the average is 13 mph. So why are the winds so modest and black spruce so ill equipped for winds?

The driving acceleration for wind speed is the pressure gradient, but the realized wind depends on latitude and that term we can't touch or feel, the almost unreal Coriolis force. The wonderful Smithsonian Meteorlogical Tables (Table 39) gives wind speeds for a range of naturally occurring pressure gradients, and for a spectrum of latitudes. At Luquillo, hurricane strength winds (72 knots) are produced by a pressure gradient of .03 mb per mile. This same storm moved to Coldfoot would have winds of only 28 knots.

Look at it this way. Luquillo trade winds, say 10 knots, are sustained by a .004 mb per mile horizontal pressure gradient. That same forcing pressure gradient at Coldfoot, Alaska would produce only a wimpy 4-knot wind. The winds up on the Arctic coast, which average 13 mph, are high because steep pressure gradients occur there associated with coastal storms especially when there is open water around.

Unlike our more temperate latitudes, Arctic coastal storms move from east to west. These arctic storms are monsters to the eye (many isobars tightly "coiled") with hurricane level pressure gradients but lesser winds. The Smithsonian Meteorological Tables don't even give pressure gradients that could make a 72 knot wind way up there.

It is to the good fortune of Coldfoot black spruce that these coastal storms don't move into the interior of Alaska with their mighty pressure gradients else the Coldfoot black spruce would need to be renamed Picea mariana horizontalis!

How about crafting some fancy index of required root holding power per degree of latitude poleward of the equator! If I were a government agency I would need to give this index an acronym like: RRHPPDLPE. Not as catchy as acid rain or global warming!

Well, what does knock over the poorly anchored black spruce? The two best candidates are summer convective thunderstorms and winds associated with firestorms. I await word from our Alaska friends on the truth about knocking over black spruce.